A rate "lock" or "commitment" is a promise from the lender to lock in a particular interest rate and a particular number of points for you for a certain period of time during your application process. This saves you from working through your entire application process and discovering at the end that the interest rate has gone up.
Rate lock periods can vary in length, between 15 to 60 days, with the longer ones generally costing more. A lending institution may agree to hold an interest rate and points for a longer span of time, such as 60 days, but in exchange, the rate (and sometimes points) will be higher than with a rate lock of a shorter period.
In addition to going with the shorter lock period, there are several ways you can attain the best rate. The more the down payment, the lower your rate will be, since you will be starting with more equity. You can pay points to reduce your interest rate for the term of the loan, meaning you pay more initially. One strategy that makes financial sense for many people is to pay points to reduce the interest rate over the life of the loan. You will pay more initially, but you'll come out ahead in the long run.
Do you have a question regarding a mortgage program?